Starmer became a barrister in 1987. He advised Helen Steel and David Morris in the McLibel case, which went to court in 1997. In an interview, he described the case as "very much a David and Goliath", and said that "There's an extremely good legal team acting for McDonalds at great expense and Dave and Helen have had to act for themselves with me as a sort of free back up whenever possible." He was also interviewed for McLibel, the documentary about the case directed by Franny Armstrong and Ken Loach.[8] He was appointed Queen's Counsel in 2002, and was joint head of his chambers, Doughty Street Chambers.

On 22 July 2010, Starmer announced the controversial decision not to prosecute the police officer Simon Harwood in relation to the death of Ian Tomlinson; this led to accusations by Tomlinson's family of a police cover-up.[10]

In the summer of 2012 Nick Cohen, a journalist, published allegations that Starmer was personally responsible for the continued prosecution of Paul Chambers, an airline passenger who, frustrated at airport delays, had posted a joke about Doncaster Sheffield Airport on Twitter. In the case known as the "Twitter Joke Trial" Chambers had been convicted of sending a message "of a menacing character". The trial and conviction provoked widespread protest by free-speech activists, but the Crown Prosecution Service maintained a long-term opposition to Chambers' appeals. According to Chambers' friends, prosecutors had been willing to stop opposing the appeals, but Starmer had over-ruled his subordinates because he was "trying to save face by refusing to admit he was in the wrong". However. the CPS said the decision was out of Starmer's hands as it was a crown court decision.[13]

In December 2013, the Labour Party announced that Starmer would lead an enquiry into changing the law to give further protection to victims in cases of rape and child abuse.[16] On 28 December Starmer said to BBC News: "well, I'm back in private practice; I'm rather enjoying having some free time, and I'm considering a number of options".[17]

Starmer married Victoria Alexander, a solicitor, in 2007, and has a son and daughter.[5]

While he was awarded a knighthood in 2014 for "services to law and criminal justice" and is therefore entitled to be known as "Sir Keir Starmer", he does not use the title.[33] He told the Ham & High, a local newspaper in his constituency that "I've never liked titles" stating that "When I was DPP, everyone called me director and I said, 'Please don't call me director, call me Keir Starmer.' It's a very similar battle now."[34]